The invention relates to friction stir welding and, more particularly, to tooling and methods for making circumferential friction stir welds on cylindrical workpieces such as tanks, rocket casings, and the like.
Friction stir welding is a relatively new technology that has been developed for welding aluminum alloys and other materials. The friction stir welding process generally involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin or spindle. Force is exerted to urge the spindle and the workpieces together, and frictional heating caused by the interaction between the spindle and the workpieces results in plasticization of the material on either side of the joint. The spindle is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing spindle cools and solidifies to form a weld.
It will be appreciated that large forces must be exerted between the spindle and the workpieces in order to apply sufficient pressure to the workpieces to cause plasticization of the material. For instance, for friction stir welding an aluminum alloy plate of xc2xc-inch thickness, forces of up to 4000 pounds or more may have to be exerted between the spindle and the plate. In a conventional friction stir welding process, these large forces are absorbed at least partially by a back-up member which engages the workpieces on the xe2x80x9cback sidexe2x80x9d of the weld opposite from the spindle. Where the workpieces have sufficient structural strength and rigidity, part of the welding forces may be absorbed by the workpieces themselves. However, in many cases the workpieces are semi-flexible structures which are incapable of supporting and absorbing the large forces involved in a friction stir welding process. Accordingly, the back-up member is usually supported by a substantial support structure.
A number of challenges are presented in friction stir welding a hollow cylindrical workpiece. Because of limited space inside the workpiece, the rotating friction stir welding tool generally must engage the workpiece from the outside and a suitable back-up tool must support the inner surface of the workpiece along its entire circumference to counteract the large inward forces exerted on the workpiece by the welding tool. A one-piece or fixed geometry back-up tool is impractical, and could not be used in workpieces in which the opening in the workpiece through which the back-up tool must be inserted is smaller in diameter than the portion of the workpiece to be welded. Thus, the back-up tool must be constructed from a plurality of members that can be placed inside the workpiece and then assembled into a full-circumference back-up tool.
The multi-component construction of the back-up tool is not optimum from the standpoint of rigidity of the tool. Tool rigidity is important because, unless the back-up tool has sufficient rigidity, the welding forces can cause deformations of the workpiece, leading to problems such as irregular welds. Accordingly, one challenge in friction stir welding hollow cylindrical workpieces is providing a back-up tool that can fit through an opening in the workpiece and can be assembled into a full-circumference back-up tool having sufficient rigidity to prevent excessive deformation of the workpiece during welding. The back-up tool should also be capable of being assembled and disassembled relatively quickly.
Another problem encountered in friction stir welding a cylindrical structure along a circumferential weld path is that the heat generated during the welding process tends to cause radial growth of the structure through thermal expansion. As a result, the welding tool tends to become buried in the weld metal, causing excessive metal flash and voiding.
Still another problem in friction stir welding a cylindrical structure relates to the rotational driving of the structure. The friction stir welding tool remains in one place and the cylindrical workpiece is rotatably driven about its axis to cause the welding tool to traverse a circumference of the workpiece. Prototype welding equipment developed by the assignee of the present application employed a rotary drive mechanism that drove the workpiece by means of an arm that engaged the workpiece and rotated about an axis coinciding with the axis of the workpiece. Thus, the torque arm of the drive mechanism was essentially equal to the radius of the workpiece. It will be appreciated that for large-diameter workpieces, the resulting torque requirement for the drive mechanism could be quite large. For instance, assuming a horizontal welding load of 2000 pounds that must be overcome by the drive mechanism, a 16-foot diameter workpiece would require a drive torque of 16,000 foot-pounds.
A further drawback of the prototype center-drive mechanism is that the drive arm tended to flex, which caused imprecise control of the rotational motion of the workpiece. For instance, at the end of a weld when the drive mechanism was stopped and the weld tool was withdrawn from the workpiece, the return of the drive arm to a relaxed condition resulted in some further rotational movement of the workpiece, causing an elongation in the exit xe2x80x9ckeyholexe2x80x9d formed by the withdrawal of the weld tool. Additionally, when starting the drive mechanism to begin a welding operation, the flexing of the drive arm resulted in some backlash such that movement of the workpiece did not begin precisely when commanded and the speed of the workpiece was not as uniform as desired. These problems were noted in welding 3-foot diameter tanks. With larger-diameter structures, the problems caused by drive arm flexure likely would be even greater.
The present invention provides tooling and methods that, at least in preferred embodiments, overcome some or all of the problems noted above. In accordance with a first aspect of the invention, a back-up tooling apparatus is provided for backing up an inner surface of a hollow cylindrical workpiece to be friction stir welded along a circumferential weld path. The back-up tooling apparatus comprises a plurality of shoes each having an arcuate shape in a circumferential direction of the workpiece, the shoes being configured to mate end-to-end with one another so as to form a substantially continuous ring having an outer diameter approximately equal to a diameter of the inner surface of the workpiece. The shoes are supported by a radially expandable support having a plurality of support arms respectively attached to the plurality of shoes. The expandable support is operable to retract the support arms and shoes inwardly to permit the back-up tooling apparatus to be inserted inside the workpiece, and is operable to extend the support arms and shoes outwardly to cause the shoes to mate with one another in end-to-end relation to form the continuous ring and to urge the ring against the inner surface of the workpiece. To enable the shoes to be retracted inwardly, alternate shoes in the circumferential direction preferably are axially staggered with respect to the remaining shoes when the expandable support is retracted.
In accordance with a preferred embodiment of the invention, the expandable support comprises at least first and second axially spaced central support members that are relatively movable toward and away from each other along an axis of the expandable support, and each shoe has support arms that are pivotally attached to the shoe and extend from opposite sides of the shoe to the central support members. Relative movement of the central support members toward each other causes the shoe to be moved radially outward and movement of the central support members away from each other causes the shoe to be moved radially inward.
The expandable support preferably comprises a screw-feed device having the first central support member threadingly mounted on a rotatable feed screw, the feed screw being rotated in one direction or another to cause the first central support member to move relatively toward or away from the second central support member. To provide the axial staggering of some shoes relative to other shoes when the shoes are retracted, the back-up tooling apparatus preferably includes a third central support member threadingly mounted on the rotatable feed screw axially spaced from the first central support member. Alternate shoes in the circumferential direction are connected via support arms to the first central support member and the remaining shoes are connected via support arms to the third central support member. Thus, when the first and third central support members are moved away from the second central support member to retract the shoes, the shoes connected to the first and third central support members are moved different distances in the axial direction.
To facilitate mating of the shoes when they are expanded outward, the mating end faces of circumferentially adjacent shoes advantageously are complementarily angled relative to the axial direction.
It is further advantageous to configure the shoes so that they overlap in the circumferential direction when the shoes are expanded to form the ring such that weld pressure exerted radially inward on the overlapping end portions causes the overlapping end portions to be forced together. The joints between adjacent shoes thus are xe2x80x9cself-sealingxe2x80x9d, which helps prevent the creation of excessively large gaps between shoes. Such gaps could result in plasticized weld metal being forced through the gaps. To further improve the connection between shoes, a seal material advantageously is disposed between mating end faces of the overlapping end portions of the shoes. The seal material can be any material that is softer than the shoes and will not melt or be forced out from between shoes under the temperatures and pressures expected to be imposed on the material during use. Examples of suitable seal material include copper and polytetrafluoroethylene.
The support and rigidity provided by the back-up tooling apparatus can be enhanced by careful construction of the shoes. More particularly, a preferred embodiment of the invention provides an apparatus wherein some of the shoes are configured to become circumferentially narrower in a radially outward direction, and the other shoes are configured to become circumferentially wider in the radially outward direction. The narrowing shoes are alternately disposed with respect to the widening shoes about the circumference so as to form an interlocking arrangement. The effect of this arrangement is that force exerted radially outward on a narrowing shoe is transmitted to adjacent widening shoes. In other words, the narrowing shoes tend to support the widening shoes, and the entire ring is thus made more rigid.
The invention in other embodiments also helps alleviate the problem of thermal growth of the workpiece. To this end, the back-up tooling apparatus preferably includes an outer restraining device configured to encircle an outer surface of the workpiece and operable to resist outward expansion of the workpiece. The restraining device advantageously comprises a pair of ring-shaped restraining bars adapted to be mounted about the workpiece on opposite sides of the circumferential weld path. Each restraining bar conveniently is formed in two or more arcuate segments that are fastened together to clamp about the workpiece.
In still other embodiments, the invention also helps alleviate the problem of inaccurate driving of the workpiece caused by flexure in the drive system. Thus, in accordance with a preferred embodiment of the invention, the back-up tooling apparatus includes a drive device in engagement with an outer surface of the workpiece and operable to rotate the workpiece about its axis. The drive device preferably comprises a cable attached to and wrapped about the workpiece and a winch connected to the cable for pulling the cable to rotate the workpiece. Preferably, the cable is attached to the restraining device that encircles the workpiece. Alternatively, a member having an outer geared surface is affixed about the workpiece and the drive device comprises at least one rotatably driven gear meshed with the geared surface for rotating the workpiece. Thus, the torque required to drive the workpiece is essentially independent of the diameter of the workpiece. Flexure in the drive device can be reduced or substantially eliminated so that more-precise control of the workpiece movement can be attained.